Summary: The immune response to polysaccharide (PS) antigens is highly regulated and has several distinguishing features including restricted subclass, variable region gene usage, fine specificity, and avidity. Simple PS not conjugated to protein (such as bacterial Levan, BL and Neisseria meningitidis group C, MCPS) elicit a thymus-independent (TI) response. PS conjugated to proteins (such as MCPS coupled to tetanus toxoid, (MCPS-TT)), on the other hand, elicit a different type of response, termed thymus-dependent (TD). We previously analyzed anti-BL antibodies and showed the importance of aromatic and basic amino acids in the CDRs of anti-PS antibodies. Our analysis of the regulation of diversity in the anti-In response mapped the Sr1 diversity gene to mouse chromosome 14. Recent studies in TCR KO mice have shown that the response to In is not only TI, but is not influenced by T cells when added back, despite a large increase in serum IgG and IgM. Previous analyses of anti-MCPS and anti-MCPS TT mAb reveal that VH gene family usage in both the TI and TD response is dominated by VHJ558. Consistent with anti-BL Abs, modeling studies also suggest a correlation of basic amino acids in the combining with an increase in affinity. BIAcore analysis of mice immunized with commercial conjugate vaccines compared with mice immunized with fixed N. meningitides showed the O-acetylation status of the PS moiety of conjugate vaccines determines the relative specificity of anti-PS Abs. Compared to immunization with fixed bacteria, the conjugate vaccines elicit a greater IgG response including antibodies to both OAc+ and OAc- PS. Furthermore, the conjugates induce higher relative avidity IgG Abs of either equal reactivity on OAc+ or OAc- or higher OAc- reactivity. Our earlier studies showed that neonatal dendritic cells are functionally impaired in their ability to present tetanus toxoid (TT) and meingococcal group C polysaccharide-tetanus toxoid conjugate (MCPS-TT) to specific T cells suggesting it could be one of the mechanisms why neonates failed to make adult-like anti-MCPS antibody responses. Therefore, studies were undertaken further to understand whether or not dendritic cells are capable of regulating the function of B cells directly. Dendritic cells isolated from MCPS-TT primed mice induced significant antigen specific proliferation of B cells in vitro. Culturing antigen primed B cells in the presence of T cells isolated from the same mice also resulted in the proliferation of B cells. The addition of dendritic cells into the immune B and T cell culture caused more robust proliferation of B cells. However, dendritic cells isolated from neonatal mice failed to have such an effect on B cell proliferation either in the presence or absence of adult immune T cells. These results suggest that dendritic cells play an important role in regulating B cell growth and responses apart from their ability to activate naive T cells, and such effect is defective in neonatal dendritic cells. Also unlike adult dendritic cells, dendritic cells from neonatal mice failed to induce IL-2 and IFN-g secretion from TT or MCPS-TT primed T cells in vitro. MCPS was found to induce IL-6 secretion from dendritic cells and the level of IL-6 produced by neonatal dendrtic cells is significantly less. In another study, characterization of T cells clones derived from MCPS-TT immunized mice revealed that polysaccharide-reactive T cell clones require antigen presenting cell contact but are not MHC restricted. One of the goals of the Children's Vaccine Initiative is to reduce the number of contacts required to immunize a child fully. To meet this goal several combined vaccines have been tested for their safety and immunogenicity in infants. We have developed a model in SW outbred mice to examine the effects of combining vaccines on the response to individual components. Groups of mice were given different combinations of Hib-TT, DTaP and IPV, in separate and mixed injections, and compared their antibody responses to different antigens to those of mice who received only a single one of these three vaccines For all Hib-containing vaccines mixed with IPV or given simultaneously in another site, anti-polio titers were reduced compared to IPV alone and the differences were greater for anti-type 1 than types 2 and 3. The differences for anti-type 1 titers were significant. Similar combinations of MCPS-TT and IPV did nor result in a diminished ant-polio response.